What you will find that many POS software out there cannot handle some hardware. So by having the equipment already, you are making problems.
It may not be the right hardware for the point of sale software, for example, anyone buying our POS system would need a computer that can run Microsoft SQL fast. Get a good computer that runs Microsoft SQL slow then if you run our software, you have wasted your money because our software will run slow.
Also, many point of sale software companies like ours have some very good deals and buying a bit here and a bit there is probably going to cost more than buying in one a big order as you are going to lose your system discount.
Almost certainly you are going to find that you are going to have to be the coordinator between different companies to get things done and you are also opening yourself to the classical problem if something goes wrong of trying to determine whose fault it is. An observation from Murphy's law states "if more then one person is at fault, no-one will admit fault."
If you really do want to buy the components separately, then I suggest that you make a decision on your software and then ask the relevant parties to discuss it between themselves as to how they can best handle it.